In modern building construction techniques, it is typical to provide a strength providing building frame, on the outside of which is mounted a curtain wall comprising panels typically made of glass, stone, synthetic stone or precast concrete. These curtain walls and the panels comprising them typically are not load bearing members and are "hung" from the building frame such that their weight is substantially supported by the building frame.
The panels are large and heavy and are generally fabricated with mounting attachments incorporated or embedded in them so that the panels can be attached to the building frame by means of bolts or other fasteners.
In particular, wall panels cast from concrete or other cementitious materials typically have concrete inserts made of steel embedded in their rear surfaces during the casting process, the concrete inserts being spaced from one another and sufficient in number to substantially resist lateral forces acting on the panel when the panel incorporating the inserts is connected to the building frame.
Typical concrete inserts comprise an elongate metal housing having a top surface to be mounted flush with the rear surface of a wall panel and a plurality of side members to extend downwardly into the concrete wall panel. The top surface of the metal housing is usually provided with an elongate, longitudinal opening, below which is received a nut in a concrete free space. The panel may for example be bolted to a structural member of a building frame by inserting the shaft of a bolt through the longitudinal opening to engage the nut in the concrete free space.
Typically, the concrete free space extends most of the length of the longitudinal opening so that the nut is slidable along the opening. Slidability of the nut along the opening helps to prevent damaging stresses from developing in the panel due to thermal expansion and contraction of the concrete panel relative to the building frame, and changes in the shape of the building structure caused by seismic activity or wind.
As may be appreciated, a concrete insert must be securely embedded in the concrete of the wall panel so that the insert cannot be withdrawn from the rear surface of the panel by lateral forces applied to the wall panel or the building structure on which the panel is installed. The ability to resist such forces is to a large extent determined by the shape of the metal housing, and in particular the shape and relative orientation of the side members of the metal housing.
It is known that when a wedge-shaped object having its larger base embedded in concrete is forcibly withdrawn therefrom, the concrete in the vicinity of the object fractures in a conical pattern emanating outwardly from the base of the wedge-shaped object to the surface of the concrete from which the object is withdrawn. The wider the base of the wedge, the larger will be the cone of fracture, and the greater will be the force required to withdraw the wedge-shaped object from the concrete.
This being the case, many concrete inserts are known having wedge-shaped metal housings. For example, U.S. Pat. No. 1,136,460 to Wright and U.S. Pat. No. 1,922,479 to Joslin describe concrete inserts having a cup-shaped metal housing wherein side members extend downwardly from all four sides of a rectangular top surface. In the insert described in Wright, all four side members extend outwardly to provide the insert with a wedge shape when viewed both longitudinally and transversely to the longitudinal slot. The insert described in Joslin is wedge-shaped only when viewed longitudinally along the longitudinal slot.
Although the wedge-shaped inserts described in Wright and Joslin may be sufficient to support a concrete panel on a building structure, the disadvantage exists that cup-like metal housings of such inserts are relatively expensive to manufacture. For example, numerous operations may be required to manufacture the cup-like housing from sheet metal, including cutting or stamping, bending, and welding.
Many presently used concrete inserts have a simple U-shaped metal housing with only two downwardly extending generally rectangular side members. Compared to cup-like metal housings, U-shaped metal housings may be more simply and economically formed by stamping or cutting the top surface and side members from a single sheet of metal, followed by bending the side members downwardly. U.S. Pat. No. 1,933,536 to Awbrey shows one variant of such an insert which has a U-shaped metal housing defining a longitudinally extending channel having open ends and an open bottom.
In the Awbrey insert, the side members are rectangular and diverge away from each other downwardly and outwardly from the top surface so that the metal housing defines a wedge shape in end view only. The present inventor has appreciated that inserts having such housings typically have lower than expected resistance to the types of forces which cause withdrawal of inserts from concrete.
The present inventor has appreciated that the primary reason for this low resistance is that, as forces are applied to pull the insert from the concrete, the diverging side members are forced to bend inwardly towards each other with the side members thus bending to slide out of the slots in the concrete containing the side members without fracture of the concrete as a cone from the innermost ends of the insert. In effect, the wedge shape formed by the side members diverging away from each other progressively collapses as the insert is drawn out of the concrete by deformation of the side members towards each other and the deformation is not resisted by any structure extending between the side members other than the bight. Use of relatively thick sheet metal for the metal housing may reduce deformation somewhat, however the use of thicker sheet metal increases the cost of the insert and is therefore undesirable. Another attempted solution, shown in Awbrey, is to provide outwardly extending rectangular flanges at the bottom of the rectangular side members to improve resistance to withdrawal forces. However, it has been found that such flanges will, like the side members, bend to slide out the slots in the concrete and are not effective at resisting withdrawal. Furthermore, formation of these flanges requires at least one additional step in the manufacturing process.
Therefore, the disadvantage exists that presently known concrete inserts having U-shaped metal housings provide insufficient resistance to forces acting on wall panels and building structures, and the steps taken to improve resistance to such forces substantially increase the cost of such inserts.
Many presently known concrete inserts have the additional disadvantage that an overly complicated mechanism is used to form a concrete-free chamber beneath the top of the metal housing and retain the nut therein. This sealing and retaining mechanism typically comprises at least three parts. A first part of this mechanism comprises a sealing member having a bottom surface and two vertical end walls, the sealing member being received in the channel to enclose the nut. A second part comprises a plastic top cap which seals the longitudinal slot. Together, the sealing member and top cap prevent concrete from entering the chamber in which the nut is retained. A third part of the mechanism comprises a spring between the nut and the bottom wall of the sealing member, which biases the nut against the top of the metal housing.
This type of mechanism has the disadvantage that a number of components must be separately manufactured and subsequently assembled, thus increasing manufacturing and material costs of the insert.